The present invention relates to an output buffer circuit useful in a large-scale integrated circuit.
Large-scale integrated circuits (LSI circuits) have output buffers that drive output terminals connected to loads in external circuits. The required driving capability of an output buffer circuit varies, depending on the total capacitance of the connected loads. If the driving capability greatly exceeds the requirements of the load, signal distortion problems such as overshoot and undershoot may occur, and unwanted electromagnetic interference may be generated by too-fast signal transitions. If the driving capability falls greatly short of the load requirements, however, the response of the output signal may become too slow to satisfy necessary timing conditions.
The driving capability of the output buffer circuits conventionally employed in large-scale integrated circuits is fixed. Not infrequently, output terminals of large-scale integrated circuits must be connected to loads that are unsuitable for the fixed driving capability. The conventional solution to this problem is to insert external damping resistors between the output terminals and loads if the driving capability is excessive, and to add external buffer circuits if the driving capability is deficient.
Unfortunately, damping resistors and external buffer circuits take up space on printed circuit boards, and add to total circuit costs. Moreover, different types of resistors and external buffer circuits must be used, depending on the specific loads to be driven. Consequently, when a circuit design is changed in a way that alters a load, additional changes in resistor values or external buffer parameters may also become necessary, creating additional work for the circuit designer and circuit manufacturer.